1. Field of the Invention
The present invention relates to a plasma CVD apparatus for forming a deposition film on a substrate, and more particularly to a plasma CVD apparatus used for forming an electrophotographic photoconductor by depositing an amorphous silicon film on a cylindrical substrate surface, and more particularly to a plasma CVD apparatus suitable for mass production.
2. Description of the Prior Art
An example of a cylindrical plasma CVD apparatus which is a typical example of prior art plasma CVD apparatus is explained.
FIG. 1 shows a schematic longitudinal sectional view of typical one of the prior art cylindrical plasma CVD apparatus. Numeral 1 denotes a cylindrical cathode electrode of a vacuum chamber, numeral 2 denotes a cylindrical base of an anode electrode which is a counterelectrode coaxially arranged to rotate around a center axis of the vacuum chamber, numeral 3 denotes a wall of the vacuum chamber, numeral 4 denotes a doughnut shaped insulation for insulating the wall from the cathode electrode, numeral 5 denotes an RF power supply, numeral 6 denotes a source gas supply pipe, arrow 7 indicates an exhaust direction, numeral 8 denotes a heater, numeral 9 denotes a motor for rotating the cylindrical base, numeral 10 denotes ground, numeral 11 denotes a vacuum gauge, numeral 12 denotes a main valve, numeral 13 denotes a leak valve, numeral 14 denotes a shield plate and numeral 15 denotes a mass flow controller.
An operation of the plasma CVD apparatus is briefly explained.
A cylindrical substrate 1 is set in the vacuum chamber and a chamber is evacuated by the vacuum system. The base 2 is heated by the heater 8 and rotated by the motor 9 to obtain a uniform temperature distribution on the base. The heater is fixed during this period. When the base temperature reaches a predetermined value, source gas is supplied from the gas supply pipe 6 to the vacuum chamber An RF voltage is applied to the cathode electrode 1 by the RF power supply 5 of 13.56 MHz, for example, while the gas is stably fed into the vacuum chamber so that a glow discharge occurs between the cathode electrode 1 and the grounded base 2 Thus, gas molecules are radical-reacted by bombardment of electrons emitted from the cathode electrode 1 to the gas molecules and deposited on the base so that a deposition film, for example, an amorphous silicon film is formed on the base 2.
By the nature of the construction of this prior art plasma CVD apparatus, the deposition film is also deposited elsewhere than on the desired area, for example, on an inner wall of the vacuum chamber wall 3 or an inner wall of the cylindrical cathode electrode 1 of the vacuum chamber. The deposition film deposited on the other than desired base area is easily removed from the surface it is on and hence is a cause of foreign material in the next deposition film forming cycle. Where the deposition films are successively formed on a number of bases exchangeably mounted, it is necessary to remove the deposition film deposited elsewhere than on the other than base area each time the base is exchanged in order to maintain a high quality of deposition film. Accordingly, continuous production is not attained by merely exchanging the base, and mass productivity is not improved. Many radicals are deposited on the surface of the electrode which faces the base. This deposition film must be removed in each film forming cycle in order to prevent the degradation of the deposition film formed on the base.